There are a wide variety of internal combustion engines in common usage today. Most internal combustion engines utilize reciprocating pistons with two or four-stroke working cycles and operate at efficiencies that are well below their theoretical peak efficiency. One of the reasons that the efficiency of such engines is so low is that the engine must be able to operate under a wide variety of different loads. Accordingly, the amount of air and fuel that is delivered into each cylinder typically varies depending upon the desired torque or power output. For throttled engines it is well understood that the cylinders are more efficient when they are operated under specific conditions that permit full or near-full load and optimal fuel injection levels that are tailored to the cylinder size and operating conditions. Generally, the best thermodynamic efficiency of an engine is found when the air delivery to the cylinders is unthrottled. However, in engines that control the power output by using a throttle to regulate the flow of air into the cylinders (e.g., Otto cycle engines used in many passenger cars), operating at an unthrottled position (i.e., at “full throttle”) would typically result in the delivery of more power (and often far more power) than desired or appropriate.
Over the years there have been a wide variety of efforts made to improve the thermodynamic efficiency of internal combustion engines. One approach that has gained popularity is to vary the displacement of the engine. Most commercially available variable displacement engines effectively “shut down” some of the cylinders during certain low-load operating conditions. When a cylinder is “shut down”, its piston still reciprocates, however neither air nor fuel is delivered to the cylinder so the piston does not deliver any power during its power stroke. Since the cylinders that are shut down don't deliver any power, the proportionate load on the remaining cylinders is increased, thereby allowing the remaining cylinders to operate at an improved thermodynamic efficiency. The improved thermodynamic efficiency results in improved fuel efficiency.
Another engine control approach is often referred to as “skip fire” control of the engine. In conventional skip fire control, fuel is not delivered to selected cylinders based on some designated control algorithm. Over the years, a number of skip fire engine control arrangements have been proposed, however, most still contemplate throttling the engine or modulating the amount of fuel delivered to the cylinders in order to control the engine's power output.
The assignee of the present application has filed a variety of applications that involve skip fire control. For example, U.S. Pat. No. 8,131,447 describes skip fire control implementations that do not require substantial throttling. As a result, various described embodiments allow for the firing of working chambers at near optimal conditions, thereby improving fuel efficiency.